Method of making a heat-resistant system

ABSTRACT

Methods of making improved electronic systems and circuits boards, and more specifically to methods of making improved electronic systems and circuits boards using heat-resistant composite materials.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. application Ser. No. 08/805,535,filed Feb. 25, 1997. I hereby incorporate by reference this pendingapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of making improved electronic systemsand circuit boards, and more specifically to methods of making improvedelectronic systems and circuit boards using heat-resistant compositematerials. Various novel heat-resistant electronic systems, circuitboards, non-segregating solid reinforcing elements, and other productsbased on these methods are also disclosed.

2. Description of Related Art

Electronic systems with modern electronic circuits components orelements are used in almost every industry including manufacturing,servicing, banking, business, financial, medical, and weaponry, as wellas in high-speed processors, cellular phones, satellite communicationsystems, deep-well equipment, jet engines, gas turbines, lap-toppersonal computers, nuclear reactors, and automobiles or othertransportation vehicles. Users of these electronic systems continuouslyrequire larger, more powerful, and faster speeds, requiring such systemsto posses and better and more processors, transistors, voltageregulators, memory, and other components.

Generally, electronic circuit components or elements are mounted bymelting and solidifying a solder metal, on plastic or ceramic circuitsboards. Metallic lead wires or lines are provided on the circuitcomponents for use in connecting these circuit components onto thecircuit boards. These connecting lines must be as few in number and asshort as possible to reduce the electrical resistances, which slow downthe speed of the electronic systems. These metallic lines must also berigid, strong, fatigue-resistant, creep-resistant, and thermallyconductive to help dissipate heat. Excessive heat generation from, e.g.,high electrical resistances, increases the system temperature, reducesthe life of transistors, and lowers the mechanical strength and creepresistance of metallic lead wires, thereby causing run-awaydeterioration of electrical and thermal resistance, temperatures,transistor life, and lead wire mechanical strengths. The degradedelectronic systems directly degrades the performance of any equipmentcontaining such electronic systems.

In many electronic systems, thermal design already is the limitingfactor. For example, to handle the heat of a high-power (15.4 W) TO-220voltage regulator operating with a 233-MHz Pentium chip presents aformidable problem that requires a proper thermal solution withoutscraping the existing mother-circuit-board architecture. Pentium chipswith even higher speeds are already in mass production.

An important consideration in the mechanical, thermal, and electricaldesign of a circuit board and an electronic system is the fact that manymaterials are used for the electronic circuit heat-resistant components,the plastic or ceramic circuit board, and the electronic system. Ingeneral, the electronic system has a metallic or plastic frame ontowhich the circuit board substrate is fixedly attached at a specificlocation thereon. The circuit board is used to electrically andphysically connect a number of circuit components together. The circuitboard substrate has a large number of through holes. Each electroniccircuit component has a number of metallic lead wires embedded into andelectrically separated by an encapsulant. All the metallic lead wires oneach circuit component extend, and point in a common direction away,i.e., vertically downward as shown in FIG. 7, from the circuit componentso that all the extending lead wires can be easily insertedsimultaneously into selected through holes at given positions on thecircuit board substrate. The inside surfaces of the through holes arecoated with specific metal layers to facilitate the wetting and bondingof the inserted metallic lead wires.

Each of the many different materials on the circuit components, thecircuit board substate, and the system frame has a specific set ofmechanical, electrical, and thermal properties, and a unique thermalexpansion coefficient. At the contact area between any two differentmaterials, there is an actual or a residual thermal expansion mismatchthat generates thermal mismatch stresses. Specifically, when theelectronic system changes in temperature, thermal mismatch stresses aregenerated between:

1) the metallic lead wires and their encapsulating plastic;

2) the metallic lead wires and the bonded metal layers on the throughholes of the circuit board substrate; and

3) the circuit board substrate and the mounting frame of the electronicsystem.

The thermal mismatch stresses usually are highly localized and can be sovery severe as to cause localized metal creep and fractures, or changesin electrical resistances and thermal conductances. Changes in theseresistances and conductances are equally, if not more, damaging thanother changes to the reliability and life of the circuit components, thecircuit board, and the entire electronic system.

Hence, the circuit boards and the electronic systems must have radicallyimproved lead wire connections, which may be fabricated by soldering,brazing, or welding methods. All these methods use molten metal alloys.Soldering metal alloys with melt temperatures below about 250° C. areemployed so that low-cost plastic circuit boards may be used. Brazingand welding metal alloys require melt temperatures respectively belowand above about 800° C. Such temperatures require ceramic circuitboards. Most such connections now are soldered joints that have:

1) high electrical resistance leading to wasteful heat generation, risein temperatures, and reduced circuit component speed and life;

2) low thermal conductivity magnifying the problems in 1); and

3) inadequate mechanical strength of the lead wire connectionsparticularly as to creep, fatigue, or shear, making all the bonded leadwires, the circuit board, or even the entire electronic system nonheat-resistant, short-lived, and unreliable.

In this invention, the above-mentioned problems of prior-art compositesare minimized by a unique, heat-resistant ceramic-reinforced compositematerial to be shown below.

For purposes of the present invention, a composite is any material thatresults when two or more materials, each having its own, usuallydifferent characteristics, are combined, giving useful properties forspecific applications.

Further, when used in the present specification, a matrix is a materialin which something is enclosed or embedded.

For purposes of the present invention uniform distribution of solidreinforcing elements in a composite matrix means that the concentrationof the solid reinforcing elements in each unit of volume, e.g., cubicmillimeter, of the solidified composite matrix is constant orsubstantially constant throughout the entire composite. In addition, acomposite has a matrix component, the matrix component is generallycharacterized by the composite component that is in the majority. Forexample, a composite made from 20% by weight solid reinforcing elementsand 80% by weight In is characterized as an In matrix composite.

Composites are important structural materials. Oftentimes composites arereinforced by suspending or embedding solid strengthening or reinforcingelements, such as, reinforcing powders, rods, sheets, weaves, orcombinations thereof within the composite matrix. Generally, the solidreinforcing elements are rigid and temperature resistant and are thusused to make the entire composite matrix more rigid and temperatureresistant. Many other benefits are achieved by reinforcing composites.For example, reinforced composites can be prepared which resist creep,fatigue, and tensile or shear fractures at temperatures which are closeto the melting point of the composite matrix.

Reinforcing elements often segregate at corners, edges, and deep butnarrow walls such as in a solder joint on a circuit board. Overcrowdedreinforcing elements at certain segregated places, such as the bottomfor heavier solid reinforcing elements or the top for lighter solidreinforcing elements, causes weakness in the composite matrix.

Reinforced composites are formed by adding solid strengthening orreinforcing elements to a liquid composite matrix followed bysolidifying or freezing the mixture to provide a reinforced compositematrix which contains the solid strengthening or reinforcing elementsembedded therein. Ideally, the solid reinforcing elements are uniformlydistributed in the composite matrix to realize and optimize the desiredperformance of the reinforced composite matrix. However, it is extremelydifficult, if not impossible, to achieve uniform distribution ofreinforcing elements in a composite matrix.

The uniform distribution of the solid reinforcing elements in a liquidor solid composite matrix is a critical factor in achieving optimumcomposite performance. If the solid reinforcing elements are heavierthan the composite matrix, they gravitationally segregate at the bottomof the liquid composite matrix during the solidification process. Thissegregation causes a non-uniform distribution of reinforcing elements inthe composite matrix. This overcrowding also reduces the efficacy ofthese solid reinforcing elements and decreases the usefulness of theresulting reinforcing composite. Solid reinforcing elements float orsegregate to the surface of the liquid matrix if they are lighter thanthe matrix.

Reinforcing elements segregation at corners, edges, and deep but narrowwalls is also very common. Overcrowded reinforcing elements at certainsegregated places, such as the bottom for heavier solid reinforcingelements or the top for lighter solid reinforcing elements, causesweakness in the composite matrix. In particular, if a composite matrixhas too many solid reinforcing elements, it may be even weaker than acomposite matrix without any reinforcement. This weakness resultsbecause the solid reinforcing elements are not sufficiently supportedby, or connected to, the composite matrix which causes localizedoverstresses, and initiates voids and cracks in the matrix. Similarly,in areas of the composite matrix where solid reinforcing elements areunderpopulated, the composite matrix is, of course, weak and notproperly reinforced.

Proper reinforcement is also problematic in cases where a composite isnarrow and deep, such as between two concentric cylinders, innarrow-clearance soldered joints on the circuit board. In this case thesolder composite thickness between the inner and outer cylindrical wallsmay be less than 1 to 5 mils. Given these parameters, the gravitationalsegregation of solid reinforcing elements at localized spots mayinitiate premature composite failures.

Thus, an inferior composite can result because of the differingdensities of the liquid composite matrix and the solid reinforcingelements. In particular, solid reinforcing elements sink when suspendedin a lighter liquid composite matrix, and float when suspended in aheavier liquid composite matrix. In either case, the solid reinforcingelements segregate due to gravity, resulting in a non-uniformdistribution of the solid reinforcing elements in the liquid compositematrix. Further, this non-uniform distribution pattern is carried overduring the composite matrix solidification, e.g., freezing or resinpolymerization of the composite matrix, resulting in undesirablesegregation patterns of the solid reinforcing elements in the resultantsolid composite matrix.

Different approaches, having varying degrees of success have attemptedto overcome the deficiencies in the prior art reinforced composites.Specifically, a tedious and time-consuming process of hand packingreinforcing elements in a composite matrix has attempted to achieve thedesired uniform distribution of the reinforcing elements. In particular,alternate sheets of composite matrix of a first thickness and solidreinforcing sheets or two-dimensional weaves of a second thickness may,for example, be hand-packed together, layer after layer, followed byliquid infiltration and freezing, pressing or thermal polymerization toform a resultant reinforced composite matrix. This process has severalshortcomings, including non-uniform distribution of the reinforcingelements caused by shifting or settling of the packed material,irreproducibility of the results packing and excessive expense informing the reinforced composite.

Another approach which has attempted to provide uniform distribution ofreinforcing elements uses a process which suspends the solid reinforcingelements in a liquid or molten composite matrix. This suspension is theninjected into and solidified in a mold causing the solid reinforcingelement to be frozen in place. However, if the reinforcing elements arenon-uniformly distributed in the liquid composite matrix prior tosolidification or if the elements settle during the solidification, thefinal distribution of these elements in the solid composite is alsonon-uniform.

Thus, what is needed then are methods of making reinforced composites inwhich the solid reinforcing elements are uniformly, or substantiallyuniformly distributed in a composite matrix resulting in a compositematrix wherein the concentration of the solid reinforcing elements ineach unit of volume, e.g., cubic millimeter, of the solidified compositematrix is constant or substantially constant throughout the entirecomposite.

In view of the prior art as a whole at the time of the presentinvention, it was not obvious to those of ordinary skill in thepertinent art how to provide for the heat-resistant, and fabricatereinforced composites for circuit board and electronic systems of theinvention.

SUMMARY OF THE INVENTION

Accordingly, a method of making heat-resistant electronic systems,printed circuit boards, and soldered, brazed, or welded joints includesmixing a liquid metal matrix having a preselected liquid density with aplurality of solid reinforcing elements to provide a mixture. Each ofthe reinforcing elements has an average density substantially equal tothe preselected liquid density thereby achieving a substantially uniformand stable distribution of the solid ceramic reinforcing elements in theliquid metal matrix. The liquid metal matrix is then frozen to preservethe uniform and stable distribution in the resultant solid composite andto make the electronic system, circuit board, or bonded jointheat-resistant. Products of the invention in various forms are alsodisclosed.

The invention accordingly comprises the features that will beexemplified in the description hereafter set forth, and the scope of theinvention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

More features and advantages of the present invention will be more fullyapparent from the following detailed description of the preferredembodiment, the appended claims, and the accompanying drawings in which:

FIG. 1 is a side elevational cross sectional view of a prior-artreinforced composite;

FIG. 2 is a cross-sectional view of a single solid reinforcing elementin the form a sphere, of a non-segregating solid reinforcing element ofthe present invention;

FIG. 3 is a side elevational cross sectional view, of an improvedreinforced composite which is reinforced with a plurality of uniformlydistributed non-segregating reinforcing elements of the presentinvention;

FIG. 4 is a cross-sectional view, of a further embodiment of anon-segregating solid reinforcing element of the present invention; and

FIG. 5 is a side elevational cross sectional view, of a furtherembodiment of an improved reinforced composite which is reinforced witha plurality of non-segregating reinforcing elements of the presentinvention.

FIG. 6 shows a top view of an electronic system; and

FIG. 7 is an enlarged side cross-sectional view of the electronic systemshowing the relative positions of the circuit components, circuit boardsubstrate, and the electronic system frame.

DETAILED DESCRIPTION OF THE INVENTION

The long-felt but heretofore unfulfilled need for a method of making animproved composite for heat resistant circuit and electronic systems isnow met by a new method of composite formation in which substantiallyuniform and stable distribution of the reinforcing elements is achieved.I use a special heat-resistant composite solder material to overcome theabove-mentioned problems.

The conventional, non-uniform distribution of reinforcing elements whichis detrimental to the performance of a reinforced composite is shown inFIG. 1, which is a cross-sectional view of a prior-art reinforcedcomposite and is denoted as a whole by reference numeral 10. As shown inFIG. 1, the prior-art reinforcing elements 11 have a non-uniformdistribution in the composite matrix 12, producing an inferiorcomposite.

The heat-resistant ceramic composite material of this invention consistsessentially of a ceramic reinforcement substantially uniformly andstabley dispersed in a liquid metal matrix of the composite material.Techniques for achieving the substantially uniform and stable dispersionor distribution of the ceramic reinforcement is described below.

As described above, the metal matrix is a bonding metallic materialgenerally an alloy of a plurality of metals and selected from the groupconsisting of a soldering material, a brazing material, and weldingmaterial.

As indicated above, soldering, brazing, and welding metallic materialsare equally useful for the practice of this invention. Their selectiondepends solely on the intended usage temperature of the entireelectronic system or the circuit board. Hence, soldering methods andsoldering metallic materials are exclusively used in this specificationfor illustration of the preferred embodiments.

Solder materials generally have poor high-temperature mechanicalproperties particularly on creep, fatigue, and hot tear. The newheat-resistant solider composite is a composite in which special ceramicreinforcing elements are stably and uniformly distributed in the matrix.This improved distribution ensures uniform spacing between thereinforcing elements and enhances load transfer among the reinforcingelements through the intervening solder matrix. Even in narrow-clearancesoldered joints, the hard and refractory ceramic reinforcing elementswill provide high creep resistance at high temperatures which normallywould fail a non-reinforced solder, or a ceramic reinforced compositewith non-uniformly distributed reinforcing elements.

For this new solder composite, specially prepared ceramic reinforcingelements are produced to insure stable and uniform distribution in aliquid solder matrix during the soldering operation. Upon solidificationof the solder matrix, a ceramic-reinforced solid solder composite isformed in which the uniformly distributed solid reinforcing elementsprovide the high creep resistance needed.

The unique composite solder will have the following benefits:

1) At high temperatures, even near the melting pint of the soldermatrix, the uniformly distributed, rigid and refractory reinforcingelements will prevent uncontrolled creep of the matrix. There will be noovercrowded or underpopulated reinforcing elements anywhere in thesolder to cause matrix failures by hot tear, fracture, or creep of thesolder matrix;

2) The new composite solder will have reliably uniform, reproducible,and enhanced mechanical properties to withstand high servicetemperatures normally considered unallowable, because of the uniformlydistributed ceramic reinforcing elements with improved load transferproperties between them;

3) The specially prepared ceramic reinforcing element, aresurface-coated with metals designed not only to be easily wetted andbonded by the liquid solder for maximum bond strength, but alsosimultaneously to minimize both surface oxidation of the coated metalsand unwanted metals diffusion during the soldering process; and

4) The new composite solder material is designed for use on conventionalautomatic soldering equipment, without major modification, in themanufacture of heat-resistant printed circuit boards and electronicsystems.

A new material and process is, therefore, urgently needed to eliminatethis very serious material and thermal problem. This problem must berapidly solved to eliminated the need for a board-level redesign whileminimizing both engineering costs and time-to-market.

The new solution in this application involves the tailor-design and useof a unique heat-resistant composite material in the soldered lead wireconnection process. Composites are materials which are not simplemetals, ceramics, and glasses or glass ceramics. For improved heatresistance, plastics are eliminated. In the process of this invention,the new composites are reinforced by suspending or embedding solidstrengthening or reinforcing elements, such as, ceramic, intermetallic,or glass reinforcing powders, rods, sheets, weaves, or combinationsthereof within the composite matrix. The solid reinforcing elements arerigid and heat-resistant and are thus used to make the entire compositematrix more rigid and heat resistant. Many other benefits are achievedby reinforcing composites. For example, reinforced composites can beprepared which resist creep, fatigue, and tensile and shear fractures attemperature which are close to the melting point of the compositematrix.

With such heat-resistant soldered connections, the solder composite willbe made heat-resistant and creep-resistant, electrically conductive toreduce heat generation, and thermally conductive to rapidly dissipatethe heat generated. Under these modifications and improvements, both thecircuit board and the entire electronic systems will become moreheat-resistant.

Through the replacement of the low-melting soldering metal alloys byhigher-melting metal alloys and the use of brazing or welding processestogether with ceramic, rather than plastic, circuit boards, the methodsand products of this invention are made even more heat-resistant.

The method of making a heat-resistant electronic system containing aplurality of electronic circuit components 49 mounted on a ceramic orplastic circuit board 42 in the system is described below. As shown inFIGS. 6 and 7, the electronic system 40 comprises a frame 41 onto whicha circuit board substrate 42 is fixed mounted and attached by, e.g.,riveting with rivets 43 as shown, at a specific location of the frame.The circuit board substrate 42 has a top major surface 44 and a bottommajor surface 45; and also a number of through holes 46 extending fromthe top major surface to the bottom major surface of the circuit boardsubstrate.

The internal surfaces of the through holes 46 areas are coated withmetallic layers 47 to facilitate their bonding to the lead wires 48 of afixed number of active or passive circuit components 49 into selectedthrough holes 46 of the circuit board substrate 42. Each of the circuitcomponent has a respective specified number of metallic electrical leadwires 48 which extend, and point in a common direction (i.e., verticallydownward) away, from the circuit component for easy insertion of theselead wires into selected ones of the through holes from the top majorsurface to at least reach a level of the bottom major surface of thesubstrate. The remaining spaces in all the selected through holes 46 ofthe substrate 42 is then filled with a heat-resistant ceramic-reinforcedcomposite material comprising solid ceramic reinforcement substantiallyuniformly and stably dispersed in a metal matrix of the compositematerial. This heat-resistant ceramic composite material further bondsall the inserted, commonly directed metallic electrical lead wires 48 tothe metallic layers 47 coated onto the respective selected through holes46.

The substantially uniformly and stably dispersed solid ceramicreinforcement in said metal matrix provides heat resistance to the bondsat least between the metallic electrical lead wires 48 and the metalliclayers 47 coated onto the respective selected through holes 46, betweenthe metallic layers in their selected through holes and the circuitboard substrate 42 and the system frame. This is so even in the presenceof the combined thermal mismatch stresses between the metallicelectrical lead wires, the coated metallic layers in the selectedthrough holes, the circuit board, and the system frame. In this way, theentire system is made heat-resistant.

These circuit boards can be mounted on a plurality of different framesof an equipment or a system such as an automobile. For instance, themodern automobile may require circuit boards mounted on different framesfor engine controls, speed controls, steering control, brake control,noise control and temperature control etc. For very high temperaturesnear the engine the circuit board/frame will require brazed joints.

Specifically, the new composite has a liquid composite matrix which hasa preselected liquid density and contains a plurality of solidreinforcing elements. Each of the solid reinforcing elements has aninner core material and an outer shell material. Specifically, the innercore material has a preselected average radius r₁, a preselected averagevolume v₁, a preselected average density d₁, and a preselected averageweight w₁, while the outer shell material has a preselected averageradius r₂, a preselected average volume v₂, a preselected averagedensity d₂, and a preselected average weight w₂. The resultingreinforcing elements have an average solid density substantially equalto a preselected liquid density of a liquid composite matrix, i.e., w₁+w₂ =(v₁ +v₂) d_(m). This condition provides a composite matrix havingsubstantially uniform and stable distribution of the solid reinforcingelements in the composite matrix. The mixture is then solidified orfrozen, while keeping the desired distribution.

FIG. 2 shows a cross-sectional view, of a non-segregating reinforcingelements of the invention and is denoted as a whole by reference numeral20. As shown in FIG. 2 the reinforcing element has an inner corematerial 21, surrounded by an outer shell material 22. An interface 23is formed between the inner core material 21 and the outer shellmaterial 22. As shown in the drawing, the reinforcing elements of thepresent invention are microcomposites themselves.

In a preferred embodiment of the present invention an improved compositeis formed using solid reinforcing elements which are designed to havethe same average density as a preselected liquid matrix, therebyminimizing or eliminating gravitational segregation. Since more commonreinforcing elements have densities different from of a matrix,reinforcing elements useful in the present invention are design to havea multi-layered or microcomposite structure.

The solid reinforcing element 20 can be any spherical powder, rod,fiber, or cylinder. For example, the solid reinforcing sphere shouldideally have a systematic, hexagonal or face-centered cubic arrangementin the liquid composite matrix. In addition, each powder should have thesame number of neighboring powders, and should be spaced at the samedistance from its closest neighbors. Further, as is known in the art, itis impossible, during formation of particular reinforcing elements, toobtain absolute uniformity in the size, weight and density of thereinforcing elements. For example, spherical powder reinforcing elementsare made of a range of powders having different sizes, weights, anddensities, which are averaged to represent an average size, an averageweight and an average density.

For illustration, a multi-layered reinforcing element in the form of aspherical powder is described. Specifically, the solid reinforcingspherical powder 20 has an inner spherical core material 21, having anaverage radius r₁, an average volume v₁ =4pr₁ ³ /3, an average densityd₁, and an average weight w₁ =4pd₁ r₁ ³ /3, where p=pi=3.142. Further,the solid reinforcing spherical powder 20 has an outer solid shellmaterial 22, having an average outer radius r₂, a preselected averagevolume v₂ =4p(r₂ ³ -r₁ ³)/3, an average density d₂, and an averageweight w₂ =4pd₂ (r₂ ³ -r₁ ³)/3, where p=pi=3.142. The non-segregatingprinciples and techniques described herein apply equally, with onlyobvious modifications, to other shapes of reinforcing elements, such asellipsoids or plates.

Improved composites are formed when the solid reinforcingnon-segregating spherical powders useful in the present invention arefreely suspended in a liquid composite matrix of density d_(m). Hence,the liquid composite matrix of volume v_(m) must have a weight of w_(m)which is:

w_(m) =4p d_(m) r₂ ³ =(v₁ +v₂)d_(m) =w_(reinforcing) sphere =4p{d₁ r₁ ³/3+d₂ (r₂ ³ -r₁ ³)/3}.

Hence, d_(m) r₂ ³ =r₁ ³ d₁ +d₂ (r₂ ³ -r₁ ³), or r₁ ³ (d₂ -d₁)=r₂ ³ (d₂-d_(m)); and

r₂ /r₁ ={(d₂ -d₁)/(d₂ -d_(m))}.sup.(1/3).

Table 1 is derived from the last equation. For example, in order tosimplify the calculation, assume that Al₂ O₃ is the solid core materialwith a density of d of 3.97 gm/cc, Bi is the solid shell material with adensity d of 9.75, and that r₁ is equal to 1 unit such as 1 micron or 1mil and the liquid composite matrix is made up of 50% by weight In and50% by weight Sn and has a liquid density, d_(m), of 7.0 gm/cc,according to the equation r₂ ={(9.75-3.97)/(9.75-7.0)}.sup.(1/3)=1.28micron or mil depending on the units.

As shown in Table 1, the reinforcing spheres are made of a rigid,heat-resistant inner core material 21, which may be any ceramic materialincluding Al₂ O₃, MgO, SiC, SiO₂, TiO₂, and ZrO₂ having densities of3.97 gm/cc, 3.6 gm/cc, 3.16 gm/cc, 2.33 gm/cc, 4.23 gm/cc, and 5.7gm/cc, respectively. Other inner core materials 21 are also useful inthe present invention including: calcium oxide and carbon.

Other shell materials 22 useful in the present invention includes: Bi,Cd, Au, Fe, Pb, Mo, Ni, Ag, W, and Co, with densities of 9.75 gm/cc,8.65 gm/cc, 19.3 gm/cc, 7.87 gm/cc, 11.4 gm/cc, 10.22 gm/cc, 8.90 gm/cc,10.5 gm/cc, and 19.3 gm/cc, respectively as shown in Table 1. The outershell material 22 of the reinforcing elements have densities greaterthan that of the 7.0 gm/cc liquid composite matrix density, whichcompensates for the lighter ceramic inner core material.

Each of the inner core ceramic material 21 of the solid reinforcingspheres given below has a lower density, d₁, than that of the liquidmetallic matrix density of the composite, e.g., 7.0 gm/cc. The outershell material 22 of the reinforcing elements has a density, d₂, whichis higher than both the ceramic inner core material density, d₁, and theliquid matrix density, of the composite. If the inner core ceramicmaterial 21 has a higher density, the outer shell material 22 must havea density lower than both the inner core material density, d₁, and theliquid matrix density of the composite. In general, the outer shellmaterial 22 must have a density d₂ which is extrapolated from the innercore material density through the liquid composite matrix density.Specifically, either d₁ >d_(m) >d₂ or d₁ <d_(m) <d₂.

The liquid composite matrix of the composites of the present inventionare, for illustrative purposes made up of 50% by weight In and 50% byweight Sn and have a liquid density, of about 7.0 gm/cc. It is desirableto select the composition of the composite matrix alloy so that itscomponent metals have their densities within 10-20%, or even 5%, of eachother, or one another at the composite processing temperature d_(m).Other pairs or groups of metals with sufficiently close densities (atthe melting point given in parentheses) include: Al(2.38 gm/cc)--Si(2.57gm/cc), Mo(5.95 gm/cc)--Ga(6.08 gm/cc), Zn(6.21 gm/cc)--Cr(6.3 gm/cc),Co(7.75 gm/cc)--Ni (7.80 gm/cc), Cd(8.0 gm/cc)--Cu(8.02 gm/cc), Ag(9.32gm/cc)--Mo(9.33 gm/cc), and Bi(10.05 gm/cc)--Pb(10.66 gm/cc).

                  TABLE 1    ______________________________________    Multi-layered Spherical Reinforcing Powders r.sub.2 /r.sub.1    ______________________________________    Values    Core Material 1:               Al.sub.2 O.sub.3                       MgO    SiC   SiO.sub.2                                          TiO.sub.2                                                ZrO.sub.2    Density (gm/cc)               (3.97)  (3.6)  (3.16)                                    (2.33)                                          (4.23)                                                (5.7)    Shell Material 2:    Density (gm/cc)    Bi         1.28    1.31   1.34  1.67  1.40  1.09    (9.75)    Cd         1.42    1.45   1.49  1.57  1.39  1.21    (8.75)    Co         1.31    1.41   1.45  1.51  1.35  1.19    (8.90)    Au         1.08    1.08   1.10  1.11  1.07  1.03    (19.3)    Fe         1.65    1.70   1.76  1.85  1.61  1.36    (7.87)    Pb         1.19    1.21   1.23  1.27  1.18  1.09    (11.4)    Mo         1.26    1.27   1.30  1.35  1.23  1.12    (10.22)    Ni         1.37    1.41   1.45  1.51  1.35  1.19    (8.90)    Ag         1.23    1.25   1.28  1.33  1.22  1.11    (10.5)    W          1.08    1.08   1.10  1.11  1.07  1.03    (19.3)    ______________________________________

The selection of In-Sn alloy as the composite matrix has a desiredeffect. Because the density of liquid In at its melting point is 7.02gm/cc, while that of the liquid Sn at its melting point is 6.99 gm/cc.The difference in liquid densities is only 0.03 gm/cc or 0.43% at atemperature between the two melting points. The density of this liquidand solid matrix alloy thus remains substantially constant at about 7.0gm/cc even with minor variations in the liquid or solid matrix alloycomposition (e.g., ±10-20% by weight of In) or even major variations inthe liquid or solid matrix alloy composition (e.g., from 0 to 100% byweight of In) due to, e.g., material preparation errors and freezingsegregations.

The radius of the inner core material 21 depends on the thickness of thecomposite, but should generally be less than 200 microns. For example,20 micron spheres having an Al₂ O₃ inner core material 21 and a radiusof r₁ =10 microns should be coated with a half-thickness of (r₂ -r₁) or2.8 microns of outer shell or surface coating material 22 of Bi with ar₂ =12.8 microns. The outer shell, of the present invention has athickness of from 1 micron to 1 mm. Such solid spherical reinforcingpowders will be stably and uniformly distributed in a liquid compositematrix alloy of 50% by weight In-50% by weight Sn and having a liquiddensity of about 7.0 gm/cc.

One major difficulty with ceramics is that it is very difficult toobtain strong voidless bonds with any coating material including metalsand ceramics. However, U.S. Pat. Nos. 5,392,982 and 5,230,924, to Li,which are incorporated herein by reference, disclose bonding methodswhich overcome various ceramic bonding problems. Hence, a coating orouter shell metal material 22 on a solid reinforcing element's ceramicinner core 21 can be selectively and voidlessly bonded to both the solidcore ceramic material 21 and to the composite matrix, to achieveefficient load transfer within each reinforcing element, betweenneighboring reinforcing elements, and to and from the reinforcingelements relative to the solid composite matrix. Further, a metallizedlayer on the ceramic inner core material 21 can form a reliable primecoat onto which other metal layers may be added, if needed. Thesesubsequent metal layers are much more easily applied, i.e., wetted toand bonded onto this properly metallized layer on the ceramic inner corethan to the ceramic core itself, thereby improving processingreliability and composite product qualities. This is because metal-metalbonding science is better understood than ceramic-metal bonding. Theliquid composite matrix can be the other metal layer which can bevoidlessly bonded onto the metal 22-coated ceramic core material 21.

The subsequent metal layers may include diffusion barriers formed ofrefractory metals such as W, Mo, Cr, Pt, and other precious metals suchas Pd, Os, Re, and the like. Further, less expensive metals such as Pb,Cu, and Fe may be used to build up the required weight and density ofthe solid reinforcing elements. In addition, protective andtemperature-resistant surface layers such as Au, Al, Cr, Au, and Ptwhich minimize surface oxidation may be used.

Solid spherical reinforcing powders each containing a solid inner corematerial and one or more outer shell or surface coating materials indifferent concentric layers can also be designed and used with equallysatisfying results in the present invention. Similar design tables forelongated reinforcing fibers or rods, sheets, weaves, or a combinationof these different shapes can also be developed. Various density data isavailable in CRC Handbook of Chemistry and Physics, D. R. Lide, Ed. InChief, CRC Press, NY 1996 which is incorporated herein by reference.

FIG. 4 shows a cross-sectional view of a further embodiment of anon-segregating reinforcing element of the invention and is denoted as awhole by reference numeral 20. As shown in FIG. 4 the reinforcingelement has an inner core material 21, surrounded by an outer shellmaterial 22. An interface 23 is formed between the inner core material21 and the outer shell material 22. An additional outer shell material22' surrounds the outer shell material 22 to minimize metal diffusion orsurface oxidation. An interface 23' is formed between the outer shellmaterials 22 and 23.

Materials other than the above ceramics, including metals or metalcompounds, are also useful in the present invention as the inner corematerial 21 of the solid reinforcing elements. In many cases, ceramicscan also be used as the outer shell or surface coating material 22.Ceramics are particularly useful because of their refractory properties,hardness, and their ability to function as diffusion barriers.

Those skilled in the art are aware that certain modifications may benecessary due, for example, to the lack of or inaccuracy of availabledensity data. Specifically, the CRC Handbook provides density values forlaboratory pure metals, not practical metals containing impurities.Indium has a density of 7.02 gm/cc at its melting point of 156.6° C.,while Sn has a density of 6.99 gm/cc at its melting point of 231.9° C.Hence, the 50% by weight In-50% by weight Sn melted alloy for thecomposite matrix at the composite processing temperature is consideredto have a liquid density of about 7.00 gm/cc. Further, the densities ofthe outer shell or surface coating materials 22, such as Bi, Cd, Co, Au,Fe, Pb, Mo, Ni, Ag, and W are given in the Handbook, but only at 20° C.and not at the actual composite processing temperature. Hence, a fewsimple experiments in some cases may be needed for precision results.

However, composites designed according to the above parameters givesubstantially uniform distribution of the suspended solid reinforcingspheres in the liquid 50% by weight In-50% by weight Sn compositematrix. For still better distribution results, the skilled person canperform a few simple tests to determine improved r₂ /r₁ values. Inaddition, when more precise and comprehensive density data for impuremetals at various composite processing temperatures become available theskilled person can even replace the estimated density values used in theabove table with the exact values by a revised calculation using theimproved density data but still according to the formula given above.

Other factors which may require consideration by the skilled artisan inpracticing the present invention include the interaction andinterdiffusion between the different materials, new phase formationswith their associated volume and density changes, and solidificationeffects due to, e.g., freezing solute segregation according to the phasediagram, and other temperature effects including those due to solutesegregation arising from freezing temperature gradients. Yet anotherfactor to consider is that the liquid composite matrix is an alloyrather than a pure metal and is therefore continuously undergoingfreezing, solute segregation, and density changes, according to itsphase diagram. Further, liquid density generally decreases withincreasing temperature.

Eutectic alloys are particularly useful in the present invention. Eachsimple eutectic alloy has the lowest melting point for the entire alloysystem, lowering the alloy freezing and composite processingtemperature. The eutectic also freezes at its singular melting point ina freezing range of temperatures as for non-eutectic alloys. Further,during freezing, non-eutectic alloys continuously changes temperature,compositions, and densities. In contrast, the eutectic alloy alwaysfreezes out at one singular composition (i.e., eutectic composition) atthe singular eutectic temperature with, therefore, one singular liquiddensity. These features eliminated or minimize changes in the liquidcomposite density due to variations in freezing temperatures and alloycompositions, freezing solute segregations according to the phasediagram, and freezing temperature gradients leading to composition anddensity gradients. A composite matrix alloy having a eutecticcomposition or nearly eutectic composition will, therefore, greatlysimplify the control of the matrix density and, therefore, the uniformdistribution of the solid reinforcing elements therein.

The uniform distribution of reinforcing elements which is achieved inaccordance with the present invention is shown in FIGS. 3 and 5, whichare cross-sectional views of inventive reinforced composites,respectively and are denoted as a whole by reference numeral 30. Asshown in FIGS. 3 and 5 the inventive reinforcing elements 20 have auniform distribution in the composite matrix 32.

The present invention minimized interdiffusion and interaction amongdifferent component materials by using surface layers of inert orhigh-temperature diffusion barriers of W, Mo, Cr, Ti, or even ofceramics such as those given as the core materials 21 listed above.Further, differing composite shape, size, and depth/width ratio (oraspect ratio) affects the temperature profile or gradient duringfreezing, local accumulation of the settling solid reinforcing elements,or depletion of liquid composite matrix metal, and their associatedchanges in alloy freezing behaviors. Again, the eutectic matrix alloywith equal or nearly equal density matrix composites described above isvery useful to achieve substantially uniform distribution of the solidreinforcing elements both in the initial liquid suspension, and in thesubsequently solidified composite matrix.

Modifications to the composite structure and the solid reinforcingelement design described fall within the scope of the instant invention.For example, the surface layer or layers may uniformly cover the entireouter surface of the solid reinforcing elements, or a heavier metal maybe partially coated onto only one side of the solid reinforcing element.In a liquid composite matrix, this partially coated solid reinforcingelement will automatically orient itself to locate the heavier, metalcoated side at the bottom therefore achieving oriented and aligned solidreinforcing elements in the composite matrix.

In addition, the ceramic core of the solid reinforcing element may besurface coated by spraying, dipping, or fluidizing with the usual W-Feand Mo-Mn ceramic metallizing composition, to be further heated to 1200°C. for the required fusion ceramic-cored solid reinforcing element.Diluted metallizing solution containing both mettallizers (W,Mo) andbrazers (Cu, Zn, Br, Fe, Mn) can produce complete adherent metal-bondedthin layers 1 micron to 100 Å with a thickness accuracy to 10-100 Å. SeLi U.S. patent application Ser. No. 08/301,582 filed Sep. 7, 1994 andherein incorporated by reference. Further, the shape of solidreinforcing elements is not necessarily limited to simple shapes such asspheres, cylinders, sheets, or weaves. Tiny structures with complicatedshapes having a number of these shapes in combination may be desirablein many cases.

The substantially uniformly and stably dispersed solid ceramicreinforcement in the metal matrix now provides heat resistance to thebonds between the metallic electrical lead wires and the metallic layerscoated onto the respective selected through holes, even in the presenceof thermal mismatch stresses only between the metallic electrical leadwires, the coated metallic layers in the selected through holes, and thecircuit board, because there is now no mismatch thermal stresses betweenthe circuit board and the electronic system frame, which is here absent.

Potential applications for various industries include high-temperatureautomobile sensors and controls, deep-well drilling equipment, jetengines or gas turbines, and electronic applications such as high-speedprocessors, cellular phones, and lap-top personal computers. Otherapplications are high-temperature engine sensors, high-power microwavesystems, power suppliers, and radiation hard equipment.

It will thus be seen that the objects set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the foregoing description withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

While the illustrated embodiments given in this specification employcertain forms of composite design and processing procedures, otherembodiments may employ other composite designs to be achieve with otherprocessing procedures. Still other alternatives in the composite designsand procedures are possible. Some techniques are provided for thealterative approaches toward fulfilling the objects of the invention.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. In the claims, means-plus-function clause areintended to cover the structures described herein as performing therecited function and not only structural equivalents but also equivalentstructures. Thus although a nail and a screw may not be structuralequivalents in that a nail employs a cylindrical surface to securewooden or other soft material parts together, whereas a screw employs ahelical surface, in the environment of fastening parts, a nail and ascrew may be equivalent structures.

What is claimed is:
 1. A method of making a heat-resistant systemcontaining a plurality of electronic circuit components mounted on acircuit board in the system, comprising:providing a frame to form a partof said system; supplying a circuit board substrate in the form of aceramic or plastic board which is fixedly attached at a specificlocation onto the frame of said system; said substrate having a topmajor surface and a bottom major surface, and also having a plurality ofwall means defining a plurality of through holes extending from the topmajor surface to the bottom major surface of said substrate; coating ametallic layer onto the respective wall means of each of selectedthrough holes on said substrate; providing a number of active or passiveelectronic circuit components a plurality of which each having arespective specified number of metallic electrical lead wires whichextend, and point in a common direction away, from said circuitcomponent; inserting said commonly directed, metallic electrical leadwires into selected ones of said through holes from the top majorsurface to at least reach a level of the bottom major surface of saidsubstrate; and filling remaining spaces in the selected through holes ofsaid substrate with a heat-resistant ceramic-reinforced compositematerial comprising a solid ceramic reinforcement substantiallyuniformly and stably dispersed in a metal matrix of said compositematerial; said heat-resistant ceramic composite material further bondingall the inserted, commonly directed metallic electrical lead wires tothe metallic layers coated onto the respective wall means of saidselected through holes; said substantially uniformly and stablydispersed solid ceramic reinforcement in said metal matrix providingheat resistance to the bonds at least between said metallic electricallead wires and said metallic layers coated onto the respective wallmeans of said selected through holes, between the metallic layers insaid selected through holes and said circuit board substrate, andbetween said circuit board substrate and said system frame, despite thepresence of combined thermal mismatch stresses between various bonded orcoated materials, whereby the entire system is made heat-resistant. 2.The method as in claim 1 wherein said heat-resistant ceramic compositematerial comprises:said liquid metal matrix having a preselected liquiddensity; and said ceramic reinforcement in the form of a plurality ofsolid reinforcing elements contained in said liquid metal matrix; andincluding forming each of said reinforcing elements to have an averagedensity substantially equal to said preselected liquid density therebyachieving substantially uniform and stable distribution of said solidreinforcing elements in said metal matrix.
 3. The method as in claim 1including choosing said metal matrix to be a bonding metallic materialselected from the group consisting of soldering material, brazingmaterial, and welding material.
 4. The method as in claim 1 wherein saidbonding metal-matrix composite material is selected from the groupconsisting of soldering material, brazing material, and weldingmaterial.
 5. The method as in claim 1 wherein solid ceramicreinforcement comprises a plurality of ceramic reinforcing elementsselected from the group consisting of powders, fibers, rods, sheets,weaves, tiny structures with complicated shapes, and combinationsthereof; andincluding mixing the plurality of said solid reinforcingelements and said metal matrix to provide a mixture having a preselectedliquid density of d_(m).
 6. The method as in claim 5 wherein each ofsaid reinforcing elements comprises an inner core material and an outershell material thereon;said shell material has an average density d₂ andan average volume v₂ such that each of said reinforcing elements has anaverage density which is substantially equal to a preselected liquiddensity of said liquid matrix metal d_(m) to thereby substantiallyuniformly and stably distribute said reinforcing elements in said liquidmetal matrix for forming a heat-resistant ceramic composite mixture andincluding:freezing said ceramic composite mixture under conditionssufficient to achieve a substantially uniform distribution of saidreinforcing elements in said frozen metal matrix.
 7. The method of claim6 wherein said inner core material has a preselected average radius orhalf thickness r₁, a preselected average volume v₁, a preselectedaverage density d₁ an a preselected average weight w₁ ; andsaid outershell material has a preselected average radius or half thickness r₂ apreselected average volume v₂, and a preselected average weight w₂ ; andwherein a plurality of said solid reinforcing elements including saidcore material and said shell material has an average solid densitysubstantially equal to the preselected liquid density of said liquidmetal matrix d_(m).
 8. The method according to claim 7 wherein w₁ +w₂=(v₁ +v₂)×d_(m).
 9. The method according to claim 7 wherein:said solidreinforcing elements are generally spherical powders; said inner corematerial has a preselected average radius or half thickness r₁ ; saidshell material has a preselected average outer shell radius or halfthickness r₂ ; and including:forming a plurality of said reinforcingelements to comprise said core material of said preselected averageweight w₁ and said shell material of said preselected average weight w₂such that said average solid density is substantially equal to thepreselected liquid density of said liquid metal matrix d_(m).
 10. Themethod of claim 7 wherein the average r₁ and r₂ are derived from theequation r₂ /r₁ ={(d₂ -d₁)/(_(d2) -d_(m))}.sup.(1/3) !.
 11. The methodof claim 5 wherein the solid reinforcing elements have a common radiusor half-thickness of from 1 to 200 microns.
 12. The method of claim 7wherein the outer shell radius or half-thickness of said solidreinforcing elements is from 1 micron to 1 mm.
 13. The method accordingto claim 7 wherein the liquid metal matrix is a metal alloy having aplurality of metal components with melting points sufficiently close toeach other or one another to significantly facilitate control of saidpreselected liquid density of said liquid metal matrix.
 14. The methodof claim 5 wherein said liquid metal matrix is a metal alloy having aplurality of metal components with melting points within 10% of eachother or one another, thereby facilitating control of said preselectedliquid density of said liquid metal matrix.
 15. The method of claim 14wherein at least plurality of said solid reinforcing elements areceramics selected from the group consisting of alumina, magnesia,silica, zirconia, calcium oxide, titanium dioxide, carbon, graphite,diamond, silicon carbide, and metal compounds.
 16. The method of claim 5wherein said liquid metal matrix is a eutectic material, therebyallowing processing of said composite material to be carried out at alow temperature.
 17. The method according to claim 7 including formingat least a selected portion of an outer surface of said outer shellmaterial to consist essentially of a metal which is much more easilywetted to the liquid metal matrix and bonded onto the solidified metalmatrix, thereby improving processing reproducibility and compositeproduct qualities.
 18. A method of making a heat-resistant circuit boardcontaining a plurality of electronic circuit components thereon,comprising:providing a circuit board substrate in the form of a ceramicor plastic board having a top major surface and a bottom major surface;forming in said substrate a plurality of wall means defining a pluralityof through holes which extend from the top major surface to the bottommajor surface of said substrate; coating a metallic layer onto therespective wall means of each of selected through holes on saidsubstrate; supplying a plurality of active or passive electronic circuitcomponents, a plurality of said components each having a respectivespecified number of metallic electrical lead wires which extend, andpoint in a common direction away from said circuit components; insertingsaid commonly directed, metallic electrical lead wires into saidselected through holes from the top major surface to at least reach alevel of the bottom major surface of said substrate; filling remainingspaces in at least a plurality of said selected through holes of saidsubstrate with a heat-resistant ceramic composite material consistingessentially of a solid ceramic reinforcement substantially uniformly andstably dispersed in a metal matrix of said composite material; andbonding with said heat-resistant ceramic composite material all theinserted, metallic electrical lead wires to the metallic layers coatedonto the respective wall means of said selected through holes; saidsubstantially uniformly and stably dispersed solid ceramic reinforcementin said metal matrix providing heat resistance to the bonds at at leastbetween said metallic electrical lead wires and said metallic layerscoated onto the respective wall means of said selected through holes,and between the coated metallic layers in the selected through holes andsaid circuit board substrate, despite presence of combined thermalmismatch stresses between various different bonded or coated materials,whereby the entire circuit board is made heat-resistant.
 19. A method ofmaking a heat-resistant circuit board containing a plurality ofelectronic circuit components thereon, comprising:providing a circuitboard substrate in the form of a plate with at least a top electricallyinsulating, ceramic, plastic, or other material layer on a top majorsurface thereof; defining on said insulating material layer a pluralityof mounting locations for mounting thereat a plurality of active orpassive electronic circuit components; coating a metallic layer at eachof the mounting locations on said substrate; supplying said plurality ofelectronic circuit components; mounting the plurality of electroniccircuit components onto the metallic layers at said defined locations onthe substrate with a heat-resistant and metal-layer wetting, compositematerial consisting essentially of solid reinforcing elementssubstantially uniformly and stably dispersed in a liquid metal matrix ofsaid composite material; and bonding all the electronic circuitcomponents with an originally metal-wetting but subsequently frozensolid, heat-resistant composite material onto the defined locations onsaid metallic layers coated onto said substrate, despite presence ofcombined thermal mismatch stresses between various different bonded orcoated materials, whereby the entire circuit board is madeheat-resistant.
 20. The method as in claim 19 including:providing aframe to form a part of said system; mounting with a mounting stress theframe onto a mounting position on said system, and wherein saidsubstantially uniformly and stably dispersed solid reinforcing elementsin said metal matrix providing heat resistance to the bonds at leastbetween the circuit components and the metallic layers coated at themounting locations on said substrate, between the metallic layers andthe substrate, and between the substrate and the frame, and combinedthermal mismatch stresses between various bonded, coated, or mountedmaterials, whereby the system is made heat-resistant.
 21. The method asin claim 19 wherein said heat-resistant elements-reinforced compositematerial consists essentially of a plurality of solid reinforcingelements uniformly and stably dispersed in said liquid metal matrix ofsaid composite material.
 22. The method as in claim 19 wherein saidheat-resistant composite material comprises:said liquid metal matrixhaving, in a liquid form, a preselected liquid density; and saidreinforcing elements are in the form of a plurality of solid reinforcingelements contained in the liquid metal matrix, and including:formingeach of the reinforcing elements to have an average densitysubstantially equal to the preselected liquid density thereby achievingsubstantially uniform and stable distribution of the solid reinforcingelements in said metal matrix.
 23. The method as in claim 19 includingchoosing the metal matrix to be a bonding metallic material selectedfrom the group consisting of soldering material, brazing material, andwelding material.
 24. The method as in claim 22 wherein solidreinforcing elements comprise a plurality of ceramic reinforcingelements selected from the group consisting of powders, fibers, rods,sheets, weaves, tiny structures with complicated shapes, andcombinations thereof; andincluding mixing said plurality of saidreinforcing elements and said metal matrix to provide a liquidsuspension mixture having a preselected liquid density of d_(m).
 25. Themethod as in claim 24 wherein each of said reinforcing elementscomprises an inner core material and an outer shell materialthereon;said shell material has an average density d₂ and an averagevolume v₂ such that each of said reinforcing elements has an averagedensity which is substantially equal to the preselected liquid densityof said liquid matrix metal d_(m) to thereby substantially uniformly andstably distribute said reinforcing elements in said liquid metal matrixfor forming a heat-resistant ceramic composite mixture andincluding:freezing the ceramic-metal composite mixture under conditionssufficient to achieve a substantially uniform distribution of thereinforcing elements in said frozen metal matrix.
 26. The method ofclaim 25 including forming at least a selected portion of an outersurface of said outer shell material to consist essentially of a metalwhich is much more easily wetted to the liquid metal matrix and bondedonto the solidified metal matrix, thereby improving processingreproducibility and composite product qualities.
 27. The method of claim25 wherein said inner core material has a preselected average radius orhalf thickness r₁, a preselected average volume v₁, a preselectedaverage density d₁, an a preselected average weight w₁ ; andsaid outershell material has a preselected average radius or half thickness r₂, apreselected average volume v₂, a preselected average density d₂, an apreselected average weight w₂ ; and wherein each of said solidreinforcing elements including said core material and said shellmaterial has an average solid density substantially equal to thepreselected liquid density of said liquid metal matrix d_(m).
 28. Themethod of claim 26 wherein w₁ +w₂ =(v₁ +v₂)×d_(m).
 29. The method ofclaim 28 wherein:said solid reinforcing elements are generally sphericalpowders; said inner core material has a preselected average radius orhalf thickness r₁ ; said shell material has a preselected average outershell radius or half thickness r₂ ; and including:forming a plurality ofsaid reinforcing elements to comprise said core material of saidpreselected average weight w₁ and said shell material of saidpreselected average weight w₂ such that said average solid density issubstantially equal to the preselected liquid density of said liquidmetal matrix d_(m).
 30. The method of claim 28 wherein the average r₁and r₂ are derived from the equation r₂ /r₁ ={(d₂ -d₁)/(d₂-d_(m))}^(1/2).
 31. The method of claim 22 wherein the solid reinforcingelements have a common radius or half thickness of no more than 1 mm.32. The method of claim 22 wherein the outer shell radius orhalf-thickness of said solid reinforcing elements is of no more than 1mm.
 33. The method of claim 22 wherein the liquid metal matrix is ametal alloy having a plurality of metal components with melting pointssufficiently close to each other or one another to significantlyfacilitate control of said preselected liquid density of said liquidmetal matrix.
 34. The method of claim 22 wherein the liquid metal matrixis a metal alloy having a plurality of metal components with meltingpoints within 10% of each other or one another, thereby facilitatingcontrol of said preselected liquid density of said liquid metal matrixd_(m).
 35. The method of claim 22 wherein at least a plurality of saidsolid reinforcing elements are ceramics selected from the groupconsisting of alumina, magnesia, silica, zirconia, calcium oxide,titanium dioxide, carbon, graphite, diamond, silicon carbide, and metalcompounds.
 36. The method of claim 22 wherein said liquid metal matrixis a eutectic material, thereby allowing processing of said compositematerial to be carried out at a low temperature.
 37. The method of claim22 wherein said shell material has a density of no more than 19.3 gramsper cubic centimeter.
 38. A method of making a heat-resistant systemhaving a mounting substrate and a plurality of components thereon,comprising:providing said mounting substrate having at least anelectrically insulating material layer on a top surface thereof;defining on said insulating material layer a plurality of mountinglocations for mounting thereat a plurality of said components; forming ametallic layer at each of the mounting locations on said substrate;supplying the plurality of said components; mounting the plurality ofsaid components onto the metallic layers with a heat-resistant andmetal-layer wetting, liquid composite mixture consisting essentially ofa plurality of solid reinforcing elements substantially uniformly andstably dispersed in a solidifiable liquid metal matrix of said liquidcomposite mixture; wetting both said metal layers on said substrate andselected portions of said components with said liquid composite mixture;solidifying said liquid composite mixture at the defined locations underconditions to maintain said solid reinforcing elements stillsubstantially uniformly and stably dispersed in a solidified metalmatrix of said composite material; individually said components,mounting substrate, metal layers, solidified metal matrix, and solidreinforcing elements being all heat-resistant for a specific service;and bonding with said solidified metal-matrix composite material theselected portions of said components onto said metallic layers on saidsubstrate, sufficiently strongly to provide said heat resistance despitepresence of combined thermal mismatch stresses between various differentbonded or coated materials whereby the entire system is madeheat-resistant.
 39. The method of claim 38 wherein said mounting stepscomprises mounting the components on the substrate with a heat-resistantand metal-layer wetting, composite material in a layer form of no morethan 5 mils thick to produce substantially uniformly and stablydispersion of the solid reinforcing elements in the liquid or solidifiedmetal matrix of said composite material even in the layer form.
 40. Themethod of claim 38 wherein said composite matrix material is selectedfrom the group consisting of a metal, and a metal alloy.
 41. The methodas in claim 38 wherein said reinforcing elements are in a substantiallyconstant but systematic geometrically arrangement and each reinforcingelement having approximately the same number of closest neighbors andbeing spaced at approximately the same distance from said closestneighbors.
 42. The method as in claim 38 including:providing a castingmold for the liquid composite material to cast thereinto; casting intothe mold said liquid composite material with the solid reinforcingelements substantially uniformly and stably dispersed therein; andallowing the liquid composite material cast into said casting mold tofreeze under conditions to maintain the substantially uniformdistribution of the solid reinforcing elements in the finished solidcomposite material to avoid overcrowded or underpopulated reinforcingelements anywhere in the bonding composite material thereby minimizingcomposite matrix failures by hot tear, fracture, or creep.
 43. Themethod as in claim 38 including providing on each of said solidreinforcing elements a surface coating to improve its wettability tosaid liquid composite matrix for improved solid composite strength. 44.The method as in claim 38 including providing on each of said solidreinforcing elements a surface coating to improve its wettability tosaid liquid composite matrix for enhanced mechanical properties so thatthe heat-resistant system board can withstand service temperaturesnormally considered unallowable because of the uniformly distributedsolid reinforcing elements with improved load transfer propertiestherebetween.
 45. A method of making a system having a mountingsubstrate and a plurality of components mounted thereon,comprising:providing said mounting substrate having an electricallyinsulating top surface; defining on said top surface a plurality ofmounting locations for mounting thereat the plurality of saidcomponents; supplying the plurality of said components; mounting theplurality of said components at said defined locations with a wetting,liquid composite mixture consisting essentially of a plurality of solidreinforcing elements substantially uniformly and stably dispersed in asolidifiable liquid metal matrix of said liquid composite mixture;wetting with said liquid composite matrix both selected portions of saidcomponents and said substrate at said defined mounting locations;solidifying the top surface of said liquid composite mixture at thedefined locations under conditions to maintain said solid reinforcingelements still substantially uniformly and stably dispersed in asolidified metal matrix of said composite material; individually saidcomponents, mounting substrate, solidified metal matrix, and solidreinforcing elements being all heat-resistant for a specific service;and bonding with said solidified metal-matrix composite material theselected portions of said components onto said substrate, sufficientlystrongly to provide said heat resistance despite presence of combinedthermal mismatch stresses between various different bonded or coatedmaterials whereby the entire system is made heat-resistant.
 46. Themethod of claim 45 wherein said composite matrix material is selectedfrom the group consisting of a metal, and a metal alloy.
 47. The methodas in claim 45 wherein said reinforcing elements being in asubstantially constant but systematic geometrically arrangement and eachreinforcing element having approximately the same number of closestneighbors and being spaced at approximately the same distance from theseclosest neighbors.
 48. The method as in claim 45 including:providing acasting mold for the liquid composite material to cast thereinto;casting into the mold said liquid composite material with the solidreinforcing elements substantially uniformly and stably dispersedtherein; and allowing the liquid composite material cast into saidcasting mold to freeze under conditions to maintain the substantiallyuniform distribution of the solid reinforcing elements in the finishedsolid composite material whereby overcrowded or underpopulatedreinforcing elements anywhere in the bonding composite material areavoided thereby minimizing matrix composite matrix failures by hot tear,fracture, or creep.
 49. The method as in claim 45 including providing oneach of said solid reinforcing elements a surface coating to improve itswettability to said liquid composite matrix for enhanced mechanicalproperties so that the heat-resistant system board can withstand servicetemperatures normally considered unallowable because of the uniformlydistributed solid reinforcing elements with improved load transferproperties between.
 50. The method as in claim 45 wherein said mountingstep comprises forming said liquid composite mixture to have, throughoutthe entire volume of the composite mixture, substantially the samenumber of said solid reinforcing elements in each unit volume of themixture;said reinforcing elements being in a substantially constant butsystematic geometrically arrangement and each reinforcing element havingapproximately the same number of closest neighbors and being spaced atapproximately the same distance from these closest neighbors.